1. Field of the Invention
The present invention relates to an aperture stop control device for a lens, and more particularly to an automatic aperture stop control system for a zoom lens system wherein any variation in F number accompanying a zooming operation will be automatically adjusted to a desired value.
2. Brief Description of the Prior Art
Two-group zoom lens systems are well-known in the photography field. In a two-group zoom lens system which has a front lens group of a negative refractive power and a rear lens group of a positive refractive power, the focal length of the system is usually variably adjusted by changing the air space formed between the front and rear lens groups. An aperture stop is usually positioned in the rear lens group and is, accordingly, moved along the optical axis upon the movement of the rear lens group relative to the front lens group during a zooming operation. This movement of the aperture stop along the optical axis causes a variation in the effective F number if the mechanical diameter of the aperture stop is fixedly held constant during the zooming operation. Therefore, the mechanical diameter of the aperture stop should be variably compensated by a predetermined value in relation to the variation in the focal length for effectively maintaining a designated F number.
In general, a cam mechanism is required for controlling the diameter of the aperture stop of a zoom lens system. The cam can provide a necessary adjustment of the aperture stop to realize a designated F number in accordance with the movement of a manually operable ring which is rotatably mounted around the lens barrel and carries the F number indications.
U.S. Pat. No. 3,889,282 discloses a device for variably compensating the mechanical diameter of the aperture stop in relation to any variation of the focal length in the zoom lens system of the above-mentioned type. However, this compensation is achieved by means of another cam mechanism which adds additional complexity, weight, etc.
U.S. Pat. No. 4,043,642 is of interest since it discloses the introduction of electric control means into a lens barrel mechanism. However, this disclosure relates to a zoom lens system having a variator lens group and a compensator lens group, and more specifically to a device for controlling the movement of the compensator lens group for a zooming operation.
A Japanese patent application has been laid open to public inspection under No. 51-45527 and is of interest in that it discloses an electric control device for an aperture stop in a zoom lens system. The Japanese patent application, teaches a combination of a first potentiometer for setting an F number, an aperture stop mechanism, a second potentiometer for detecting the diameter of the aperture stop and means for driving the aperture stop mechanism until the output of the second potentiometer coincides with that of the first potentiometer. The output of the second potentiometer is shifted by a predetermined value by means of switches to accommodate any change in a relay lens of the zoom lens system to shift the variable range of the focal length. The number of switches are increased if the number of exchangeable relay lenses is increased. The Japanese patent application neither relates to a zoom lens system in which a variation in effection F number accompanies a zooming operation, nor does it disclose any means for compensating the mechanical diameter of the aperture stop in relation to the zooming operation.